Aqueous coating compositions for the preparation of protective or decorative coatings have the advantage of being non-flammable, nonpollutant, and inexpensive, owing to the absence of organic solvents, i.e., aliphatic and aromatic hydrocarbons, organic esters or ketones, etc. These compositions are of two general types (1) dispersions of resinous materials which ultimately form the protective or decorative coatings, and (2) resinous binders with polyelectrolyte structure. Dispersions without polyelectrolyte structure consist of fine particles of resin and additives distributed in the aqueous phase, emulsifiers being necessary as aids for the distribution. The dispersions with a solids content of 50 percent have nearly the same viscosity as water and do not behave anomalously when further diluted with water. Coatings, however, prepared from dispersions of binders offer only low resistance to corrosion and, thus, corrosion protection, owing to the inclusion of the low molecular weight emulsifiers. Therefore, they are most commonly used in interior paints and decorative coatings; or in paper coatings and textile sizings. They have only limited use for coatings on corroding metals.
Water-dilutable binders with polyelectrolyte structure carry a number of saltlike groups in the macromolecule (soaps) and, contrary to dispersions, form colloidal or true solutions with water. The coatings obtained from such binders, optionally in the presence of polyvalent cross-linking resins (phenolic resins, amine resins, etc.) provide excellent corrosion protection of metals. Such binders consist of partial condensation products of resin-like polycarboxy compounds and polyhydroxy compounds in a molar ratio of about 1 : 1 and are, e.g., described in Austrian Patent Nos. 267,185 and 268,675. A practically identical product and procedure are described in Deutsche Offenlegungschrift No. 17 69 129 according to which carboxy reactive resin components are partially esterified such that a polar-uniform product is obtained which is recommended for electrodeposition without co-employing polyvalent cross-linking resins.
Despite the known aforesaid advantages, the use of water-soluble binders with polyelectrolyte structure is restricted due to their peculiar behavior when diluted with water to a sufficiently low viscosity as is necessary for application by brushing, dipping, or spraying. Unlike solvent dissolved binders, with water-diluted binders the viscosity does not continuously decrease with increased diluent, but remains constant over a wide range of concentrations or will even rise in some cases. With high quantities of water and correspondingly low solids, the viscosity decreases rapidly within a very narrow range of concentrations (about 30-25 percent), and finally reaches almost the viscosity of water (E. Huttmann et al, Plaste und Kautschuk 17, 202, 1970). This anomalous viscosity characteristic specific to the known water-dilutable binders entails a number of serious disadvantages, ruling out a general use of such binders including
1. The low binder solids concentration at application viscosity causes low film thickness of the dried cured coating and necessitates application of more than one coat in order to obtain a sufficiently thick and protective film; PA1 2. The evaporation of such high quantities of water from the wet film takes long flash-off periods or an increased demand of heat energy. The presence of higher quantities of water in the film at the beginning of the cure leads to sizzling, cracking, splitting, or bursting of thick films; and PA1 3. The steep reduction in viscosity within a narrow range of concentrations requires special precautions when adjusting the viscosity to application level, since the slightest deviations cause great fluctuations in viscosity.
The aforesaid factors make it difficult to work with water-dilutable binders, practically precluding their industrial use. Therefore, watersoluble binders have been mainly used up to now in electrodeposition coatings which require binder solids concentrations of less than 15 percent -- beyond the critical range.
It is possible to bridge or reduce the aforesaid disadvantages on the application of water-soluble coating compositions by co-employing substantial quantities, i.e., above about 50 percent, of water-tolerant organic solvents, such as alcohols, ketones, or glycol ethers, and thereby obtain a dilution curve which is siimilar to that of solvent dissolved binders. Although such measures enable water-diluted binders to be applied, the special advantages of the waterborne binders are set off by the addition of the solvents.